1. Field of the Invention
This invention relates to electroacoustic apparatus, particularly to bulk mode acoustic delay devices.
2. Description of the Prior Art
In the prior art, multiple reflections from bulk acoustic waves in delay lines have caused undesirable output signals. A bulk mode acoustic delay line having a transducer at either end of a substrate provides a desired output signal after a bulk acoustic wave is generated and travels from the input transducer to the output and an undesired triple transit signal after the bulk acoustic wave is reflected at the output and travels back and forth through the delay line to the output again. The triple transit output causes an interference ripple in the bandpass of the delay line corresponding to the difference in the delay between the direct output and the triple transit output. The interference period for a triple transit output is .DELTA.f=1/2.tau. where .tau. is the delay time of the delay line. One example of a bulk acoustic delay device for delaying signals and for providing triple transit suppression is described in U.S. Pat. No. 4,099,147 issued on July 4, 1978, entitled "Bulk Acoustic Delay Device" by Bruce R. McAvoy and assigned to the assignee herein.
In U.S. Pat. No. 4,099,147 the input transducer aperture provides a predetermined diffraction in the bulk acoustic wave launched causing the bulk acoustic wave to spread laterally as it propagates through the medium. The bulk acoustic wave impinges upon a receiving transducer to provide a direct output. The bulk acoustic wave after traversing the delay line three times impinges upon the receiving transducer and an additional transducer spaced outwards to intercept only the triple transit wave. The triple transit output is attenuated due to a difference in the acoustic path lengths from the input to the two receiving transducers to provide a phase difference of the two received signals. Bulk acoustic waves traveling through the medium also exhibit path attenuation due to the distance the wave travels from the input transducer which also reduces the triple transit signal.
An alternate approach to provide triple transit suppression in a bulk acoustic delay line is described in a paper entitled "Wideband Microwave Acoustic Delay Line With Exceptionally Smooth Phase and Loss Response" by W. R. Sperry, E. P. Kirchner and T. M. Reeder, published in the Digest of the International Microwave Theory and Technique Symposium sponsored by the IEEE on May 17, 1971. In the paper, triple transit supression is enhanced by using an angled end face to place the null in a radiation pattern of the triple transit signal, launched at the input transducer aperture, at the position of the output transducer aperture. In other words, the beam axis of the bulk acoustic wave launched by the transducer is tilted or directed such that on the third transit the main beam energy misses the receiving transducer and only the side lobes are seen. The beam axis tilting approach is useful only where the input and output transducers are in the far zone relative to each other so that the acoustic beam is in the Fraunhofer field region at the output transducer.
The use of additional receiving transducers in a surface acoustic wave delay device to provide triple transit suppression by means of phase interference has been described in a paper entitled "Triple-Transit Suppression in Surface Acoustic Wave Devices" by M. F. Lewis, published in Electronic Letters, Volume 8, No. 23, pages 553-554 on Nov. 16, 1972. In FIG. 2, two dummy interdigital transducers are spaced an additional distance of .+-..lambda./4 to reflect back to the input transducer two signals which are .+-.180.degree. out of phase with the reflection of the wave from the output transducer. The reflected waves are canceled at the input transducer leaving no surface acoustic wave to be reflected by the input transducer to become the triple transit signal at the output transducer. Nulling the reflected signal at the input transducer will not provide triple transit suppression in bulk mode acoustic delay lines.
Another approach to reduce the triple transit signal is described in U.S. Pat. No. 3,942,139 issued Mar. 2, 1976 entitled "Broadband Microwave Bulk Acoustic Delay Device" by Herbert Warren Cooper and John de Klerk and assigned to the assignee herein. In U.S. Pat. No. 3,942,139 an input and output transducer are placed side by side on one surface of a substrate. A bulk acoustic wave is launched from the input transducer and travels through the substrate whereupon it is reflected by the lower surface and travels back impinging upon the area of the input and output transducers whereupon the output transducer generates a signal. The triple transit signal actually the double transit signal, travels back and forth through the substrate and impinges upon the output transducer. The double transit signal is attenuated by propagation loss in the substrate medium and by diffraction loss or spreading of the bulk acoustic wave which propagates far enough away from the input transducer to be in the Fraunhofer region.
However, if the desired delay is short, such as 200 nanoseconds, the bulk acoustic wave will travel 1.217 millimeters in the substrate medium such as quartz until impinging an output transducer. The beam spreading of the ultrasonic elastic wave is provided for after propagating a certain distance by making the aperture or width of one side of the transducer d small such that the relation 2d.sup.2 /.lambda. is greater than distance L where L is the acoustical path distance that the bulk wave is in the Fresnel zone. The beam spreading may diverge at the angle .alpha. where sin .alpha.=.lambda./2d. In other words, diffraction occurs sooner and is increased in angle .alpha. as the dimension d for one side of the transducer is decreased.
The Fresnel zone extends from the input transducer approximately out to a distance L where L=2d.sup.2 /.lambda. where d is the length of one side of the transducer for rectangular transducer and .lambda. is the wavelength of the ultrasonic elastic wave in a transmission medium. .lambda. may be determined by the relationship .lambda.=V/f where V is the velocity of the ultrasonic wave in the transmission medium and f is the frequency of the ultrasonic elastic wave. In the Fresnel zone, the bulk acoustic wave propagates away from the transducer maintaining a cross section corresponding to the transducer. In the Fraunhofer field region, the bulk acoustic wave appears to have originated from a single point instead of from the overall area of the transducer.
It is therefore desirable to provide electroacoustic apparatus for delaying signals and for providing wideband triple transit suppression in bulk acoustic delay lines about a predetermined center frequency.
It is further desirable to provide bulk acoustic wave delay lines that can delay signals of an extremely broadband width such as an octave bandwidth with triple transit suppression for delays of 0.5 microseconds or less.
It is further desirable to provide bulk acoustic wave delay lines having large input and output transducers which generate and receive bulk acoustic waves in the near or Fresnel zone and where the topology of the transducers have various acoustical path lengths therebetween causing phase and amplitude cancellation which reduces the triple transit signal.